1. Technical Field
The invention involves a lead-free lubricating material with a sintered matrix on a Cu or CuSn base and a solid lubricant. The invention also involves a sinter powder for manufacture of the same, a lubricating compound material with a steel supporting layer, and a bearing metal layer of such a sinter-fused lubricating material and a lubricating element.
2. Related Art
Lead-free sintered lubricating with materials made of copper or on a copper and tin base, in particular on the basis of a bronze matrix, are known for their good conductivity of heat and their high resistance to wear and tear and corrosion in comparison to similar materials containing lead. The development of such materials was based on the desire to replace lubricating materials containing lead, since lead has been classified as a material harmful to the environment. Since lead in materials has the function of a solid lubricant, a substitute material must be found for it. Otherwise, for example, a one-phase bronze material lacking a lubricating soft phase would show an increased tendency to seize up under mixed friction conditions. From this point of view, in the literature and in practice a series of various combinations has been studied and put into use.
Aside from lead, the most commonly used solid lubricants are molybdenum disulfide (MoS2) and graphite, which are also used in other lubricating coatings based on another matrix material as solid lubricants.
For example, in U.S. Pat. No. 6,613,453 B2, a sintered lubricating layer on a copper base with a tin content of 7 to 13 weight %, a silver content of 0.7 to 2 weight %, and optionally up to 0.5 weight % of molybdenum disulfide and optionally up to 2 weight % graphite have been suggested as solid lubricants.
EP 0 224 619 A1 treads a different path by suggesting improvement of resistance to corrosion by reducing the lead content in a copper alloy and at the same time providing a bismuth proportion of 5 to 25 weight %. In getting rid of lead, a preferred proportion of bismuth of 12 to 20 weight % and a proportion of tin of 1 to 2 weight % have been noted.
Starting from this point, DE 10 2004 011 831 B3 suggests a sintered lubricating material with 10 to 15 weight % tin, 0.5 to 10 weight % bismuth, 1 to 12 weight % graphite and the rest copper. By adding graphite and with an increased proportion of tin, the amount of bismuth can be reduced and the cost of the lubricating material is thereby reduced. Moreover, this compound permits the complete removal of lead and offers excellent tribological characteristics. To be sure, tin, which is used in this material as a binding material for the solid lubricant graphite, is a comparatively expensive alloy material. Reducing the binding material tin and at the same time reducing the lubricating material bismuth does not seem possible.
Moreover, it should be noted that bismuth like lead has a low melting point, though in general it is harder and more brittle than lead. The lubricating materials graphite and molybdenum disulfide are also problematic. They form compounds with the steel of the supporting layer and in certain circumstances with the material of the expellers and thus influence the flow characteristics of the bearings.
In particular with high loads and high temperatures, as may appear in plain bearing or bush applications, in particular in gearboxes and combustion engines, carbon or molybdenum disulfide as solid lubricants in sintered materials show insufficient chemical durability. This negative characteristic is moreover strengthened by the fact that with an increasing amount of tin the heat conductivity of the matrix drops, and the bearing temperature increases as a result.
Hexagonal boron nitride (h-BN) is also known as a solid lubricant. For example, in DE 197 08 197 it is suggested to add hexagonal boron nitride to a proportion of 0.1 to 3.5 weight % plus 0.1 to 3/5 weight % graphite in the form of a powder mixture to a matrix material on an iron base. The powder mixture is then thickened and under contact with copper or a copper alloy is sintered, whereby the copper or the copper alloy is infiltrated into the pores of the sinter body.
The use of hexagonal boron nitride as a soft component or as a solid lubricant is also known from the article “Wear mechanism of Ni—P—BN(h) composite autocatalytic coatings” by Leon et al., published in Surface & Coatings Technology 200 (2005), 1825-1829. In this article, a catalytically applied lubricating layer on a nickel-phosphorus base is described and examined for its tribological characteristics.